This invention relates to a ternary intermetallic compound capable of reversibly sorbing hydrogen, and to a method of storing hydrogen using the compound. More specifically, this invention relates to a ternary intermetallic compound capable of reversibly sorbing hydrogen, in which the decomposition pressure of the compound as a hydride at a given temperature can be controlled.
The use of hydrogen gas as a working fluid in a closed system to provide for space heating, space cooling and power generation is under active consideration today as means for utilizing low-grade sources of heat to conserve on fossil fuels. One such system is described in U.S. Patent Application No. 773,363, filed Mar. 1, 1977 and assigned to the common assignee. The system described, therein operates as a chemical heat pump by utilizing the ability of metal hydrides to chemically store hydrogen at a relatively low temperature and pressure in a concentrated form and then release the hydrogen at an elevated temperature and pressure. In order for systems such as these to effectively utilize low-grade thermal energy such as waste heat from fossil or nuclear power plants or renewable thermal energy sources such as sunlight, it is imperative that the metal hydrides have sufficient decomposition or desorption pressures at the decomposition temperatures which are attainable from the available sources of thermal energy. Ideally, it should be possible to modify a hydride to decompose at the temperatures available to provide hydrogen gas with sufficient pressure to react with a second hydridable metal to form a second hydride for thermal energy storage.
Binary intermetallic compounds of particular interest for this purpose have the general composition AB.sub.5 where A is calcium, lanthanum or a rare earth and B is nickel, cobalt or a 3d transition metal, since these compounds have good hydrogen-desorption kinetics and large hydrogen storage capacity. The binary hydride LaNi.sub.5 H.sub.6.7 in particular has been investigated. However, in many applications, materials are required whose decomposition pressures are different from those of LaNi.sub.5 H.sub.6.7. The desorption pressure of this compound may be modified by substituting either lanthanum or nickel with other elements. It is known that 20% substitution of nickel by a number of transition metals can lower the decomposition pressures by a factor of about 4.